We are trying to plan a vacation to FL in December during Christmas break. My husband thinks we should stay in the Keys (Marathon) because it will be warmer. Is there that big of a difference as far as temp goes between southern FL and the Keys during the winter? We would like to be able to swim and enjoy the beach with the kids (just like it was summertime). We would also like it to be a family friendly place. Any places you could recommend that stay warm enough? Thank you.